- Move sentinel from -1 to -100 (outside the negate range) so moves from position 0 no longer collide with negated row at -1 - Pull count check + position validation inside sql.begin in POST so two concurrent inserts can't both pass the max-5 guard - Wrap movePane + state UPDATE in a single transaction in PATCH so partial failures roll back consistently Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>